CFD STUDY OF SUBMERSIBLE MIXERS IN ANAEROBIC DIGESTERS

摘要

A computational fluid dynamics (CFD) technique was applied to study flow generated by submersible mixers in a full-scale anaerobic digester. Each mixer has three main parts consisting of a support frame, a two-blade impeller, and a shaft. The mixers were operated at low speeds to stir biodegradable materials that exhibit Newtonian or non-Newtonian behavior at 0%, 2.5%, 3.5%, 5.4%, 7.5%, 9.1%, and 12.1% total solids (TS), in which a Herschel-Bulkley fluid at TS = 3.5% exhibits shear-thinning or shear-thickening behavior depending on the power-law index. The commercial CFD code Fluent 15.0 was used to solve the steady-state flow where the impeller rotation was characterized by a multiple reference frame approach. The velocity field and velocity gradient were plotted to analyze mixing at all TS levels. The results demonstrate that a directly opposite position for each impeller obtains the highest average velocity, and that the mixing energy level increases nonlinearly with an increase in TS. Further, an in-depth discussion about viscosity impacting flow fields, evaluation of mixing performance, application of velocity gradient, and factors that influence mixing energy is conducted.